Humectants draw moisture to them, which generally is a good thing. In normal and high dews, they will draw moisture from the air to them, adding moisture to your hair. However in low dews, there may be more moisture in your hair than in the air, so the humectants will suck moisture out of your hair instead.

Anti-humectants basically seal your hair, so the moisture in it cannot escape. However, that seal blocks outside moisture from getting into your hair too.

Pretty much all of my hair products have glycerin or some other humectant in them. Someone here suggested sealing with an anti-humectant (such as olive oil or grapeseed oil) after using my rinse out conditioner but before applying styling products to seal in the moisture from my rinse out and seal out the glycerin so it can't suck moisture from my hair.